The use of sliding contacts, or brushes, as current conductors between an externally connected circuit and the rotating part of a D.C. motor, i.e., its commutator, is common practice today. Nevertheless, such brushes are usually an undesirable feature because of maintenance problems such as replacing worn or broken brushes.
In a typical D.C. motor the brushes are held in contact with the commutator by means of brush springs. Typically, these springs are single coil compression springs placed in line with the brushes. Each spring engages one end of a brush and exerts pressure on the brush in the direction of the commutator.
One problem with this arrangement is that the length of a brush, and therefore its life, it limited by the position of the single coil compression spring with respect to the brush. Another problem is the uneven pressure exerted by a coil spring over the life of a brush caused by the spring moving from a compressed state to an extended state as the brush wears over time. This change in pressure prevents the optimization of the rate of brush wear so as to achieve longer brush life and, when the spring is extended, good vibration response characteristics.
Improvements have been achieved as to some of these problems through the use of constant force type springs. However, in applications using motors in the low integral horse power range, the principles of constant force spring arrangements have not been practiced due to the cost of such arrangements and the problems associated with brush replacement.
The cost of single coil constant force extension spring assemblies tends to be more than the cost of comparable single coil compression springs. This is mainly due to the cost of connecting the extension spring to a backer plate used to make the brush replaceable. In contrast, the cost of double coil constant force extension springs tend to be less than the cost of single coil compression springs because such extension springs have been traditionally used without backer plates. However, this lower cost advantage is offset by the fact that traditionally replacement of the brushes requires removing a motor's commutator, which is generally impractical once the motor has been placed in field service.